1) Field of the Invention
The present invention relates to an optical node and an optical add/drop multiplexer for connecting a plurality of networks, which can transmit an optical signal as it is (without conversion to an electric signal) and can continue communication even when a failure occurs in the networks.
2) Description of the Related Art
It is required for a conventional metro-system to meet the demand for high reliability, and quickly recover from a failure such as optical fiber breaking. For example, the recovery must be performed within 50 milliseconds according to the Synchronous Optical NETwork/Synchronous Digital Hierarchy (SONET/SDH) standard, which is an international standard for a high-speed digital communication system. As an optical protection system, Optical Unidirectional Path Switched Ring (OUPSR) and Optical Shared Path Protection Ring (OSPPR) have been suggested and commonly used.
FIG. 9A is a diagram of a ring network E in the conventional OUPSR system. The ring network E has a redundant configuration. That is, the ring network E includes a working line 10, a protection line 20, and a plurality of optical add/drop multiplexer (OADM) nodes 30a to 30d. 
Each of the OADM nodes 30a to 30d includes a transponder 31, an OADM switch unit 32a for the working line 10, an OADM switch unit 32b for the protection line 11, and a per-channel optical switch 33 (for example, see Published Japanese Translation of PCT international publication for patent application H11-508428). In a typical OUPSR system, a sending terminal includes an optical coupler 33′, while a receiving terminal includes a per-channel optical switch 33.
For example, the optical coupler 33′ of the OADM node 30a (sending terminal) branches and transmits an optical signal from the transponder 31 thereof to both of the working line 10 and the protection line 20 via the OADM switch units 32a and 32b. However, only the optical signal transmitted through the working line 10 is received by the transponder 31 of the OADM node 30c (receiving terminal), since the per-channel optical switch 33 thereof is basically connected to the OADM switch unit 32a for the working line 10.
FIG. 9B is a diagram of configurations of the conventional OADM switch units 32a and 32b, which includes an optical multiplexer/an optical demultiplexer, such as an arrayed waveguide (AWG), and per-channel optical switches. FIG. 9C is a diagram of configurations of the conventional OADM switch units 32a and 32b, which includes wavelength-selective switches (WSSs) whose development is advanced in recent years (for example, see W. J. Tomlinson, “Wavelength-selective switching architecture and technology overview”, OFC 2004, WC3, Optical Society of America, February, 2004).
FIG. 9D is a diagram of a recovery operation from a failure in the OUPSR system. When a failure 11 occurs between the OADM nodes 30a and 30b, the per-channel optical switch 33 of the OADM node 30c (receiving terminal), which has been connected to the OADM switch unit 32a for the working line 10, is connected to the OADM switch unit 32b for the protection line 20. As a result, the optical signal transmitted from the OADM node 30a (sending terminal) via the protection line 20 is received by the transponder 31 of the OADM node 30c (receiving terminal) (for example, see Hiroyuki Kasai and other 3, “Easy understandable SDH/SONET transmission system”, Version 1, Ohmsha, April, 2001, pages 110 to 118).
The OADM node 30c can detect the failure 11 by monitoring optical power level by a photodiode (PD) or the like in the transponder 31 thereof. The OADM node 30c can receive failure information, such as an alarm indication signal (AIS), via an optical supervisory channel (OSC) from an OADM node just after the occurrence point of the failure 11 (in FIG. 9D, the OADM node 30b) that is monitoring passing-through optical signals.
On the other hand, FIG. 10A is a diagram of a ring network F in the conventional OSPPR system. The ring network F includes a first working line 15 and a second working line 25, instead of the working line 10 and the protection line 11 shown in FIG. 10A. The first working line 15 is used for transmission of optical signals with the highest priority. Utilization efficiency can be improved by providing the two working lines 15 and 25.
Each of the OADM nodes 35a to 35d in the ring network F includes two transponders 36a and 36b, two OADM switch units 37a and 37b, and a per-channel optical switch 38. The transponder 36a and the OADM switch unit 37a are for the first working line 15, while the transponder 36b and the OADM switch unit 37b are for the second working line 25. The per-channel optical switch 38 switches between the first working line 15 and the second working line 25.
The transponder 36a of the OADM node 35a (sending terminal) transmits an optical signal to the OADM node 35c (receiving terminal) through the first working line 15. Simultaneously, the transponder 36b of the OADM node 35a can transmit an optical signal to the OADM node 35d (receiving terminal) through the second working line 25. Furthermore, the OADM node 35c (sending terminal) can transmit an optical signal to the OADM node 35d (receiving terminal) through the first working line 15 if it is not saturated.
FIG. 10B is a diagram of a recovery operation from a failure in the OSPPR system. When the failure 11 occurs between the OADM nodes 35a and 35b, the per-channel optical switch 38 of the OADM node 30c (receiving terminal), which has been connected to the OADM switch unit 37a for the first working line 15, is connected to the OADM switch unit 37b for the second working line 25. Similarly, the per-channel optical switch 38 of the OADM node 35a (sending terminal), which has been connected to the OADM switch unit 37a for the first working line 15, is connected to the OADM switch unit 37b for the second working line 25.
Thus, transmission is temporarily performed via the second working line 25 when a failure occurs. Communications with a low priority performed on the second working line 25 before the failure 11 is disconnected (for example, see Noboru Yajima and et. al. “Fujitsu FLASHWAVE 7500”, OPTRONICS, Optronics Corp., August, 2002, pages 158 to 161).
The failure 11 is detected by the OADM node 35b just after the occurrence point of the failure 11. The OADM node 35b transmits the failure information, via the OSC of the first working line 15, to the OADM nodes 35c, 35d, and 35a in the order of the transmission direction.
On the other hand, FIG. 11 is a diagram of a plurality of ring networks in which an optical signal is converted to an electric signal. A node 45 between a ring networks G and H converts an optical signal transmitted over optical fibers 10 and 20 in the ring networks G and H to an electric signal. An electric switch 46 of the node 45 switches wavelength, route, and the like, based on the converted electric signal. The above configuration has various problems such that a transmission capacity is limited, cost and size of the node 45 is increased, a signal format is fixed, and so on. Therefore, it is required to connect a plurality of ring networks, each of which has the above protection function, by an optical signal.
FIGS. 12A and 12B are diagrams of functions of a WSS, FIG. 12C is a perspective view of the WSS, and FIGS. 12D and 12E are side views of the WSS. Such a WSS is disclosed in Published Japanese Translation of PCT international publication for patent application 2003-515187.
A WSS 1200 is a switch that can output a signal of an arbitrary wavelength in an input wavelength multiplexed signal to an arbitrary output port. The WSS 1200 includes 1 input port and N output ports as shown in FIG. 12A, or N input ports and 1 output port as shown in FIG. 12B.
As shown in FIG. 12C, the WSS 1200 includes a spectroscopic element 1201, an optical input and output port 1202, light-converging elements such as lens 1203, and a movable reflector array (a mirror array) 1204 for each wavelength. The spectroscopic element 1201, which is a diffraction grating, disperses wavelength division multiplexing (WDM) lights in different directions (along Z direction in FIG. 12C) for respective wavelengths. Lights along an angular dispersing direction spread in an X-Z plane. Correspondingly, a plurality of movable reflectors (MEMS mirrors) is provided in the movable reflector array 1204 along a dispersing direction (a lateral direction in FIG. 12C).
As shown in FIG. 12E, an incident light from the input port (IN) can be input to any one of the output ports (OUT) for each channel, by changing an angle of the movable reflector array 1204 along an arrangement direction (in Y direction in FIG. 12E) of the ports.